Questions still surround the 2009 disappearance of Utah mother Susan Powell ever since the suicides of her husband, Josh Powell, and brother-in-law, Michael Powell, who authorities believe were "directly involved in her disappearance."

Although the brothers were considered persons of interest in her disappearance, there was never enough evidence to charge either one of them, said West Valley City Deputy Police Chief Mike Powell. The evidence against them was "circumstantial," he told ABC News.

Despite the scarcity of information to go on -- no crime scene, no body -- police said they were still working leads to find out what happened to Susan Powell.

"It's paramount that people understand that it's not a closed case," the deputy police chief told ABC News. "This is still an open investigation, and we will continue to pursue any information provided to us actively and with just as much vigor."

Some hope lies in Steven Powell, Susan Powell's father-in-law, who police believe might know something about her disappearance. Convicted on charges of voyeurism and child pornography, Steven Powell is currently in custody at Monroe Corrections Center in Monroe, Wash.

"Susan is still missing," he said. "If Steven Powell has any information or indication that he knows where or what may have happened, that would be important for us to have.

"Whether or not we're able to obtain that, therein lies the difficulty," he told ABC News.

Susan Powell, 28, was last seen in December 2009 at the Utah home she shared with her husband and their two young sons. Her husband told authorities that he had taken an impromptu midnight camping trip with the boys -- in the midst of a winter storm -- the night his wife vanished.

Josh Powell, 36, said that he returned home to find his wife gone and continued to state that his wife had left on her own.

Susan Powell's disappearance triggered a massive investigation that focused on her husband, who killed himself and his two sons in a fiery explosion at his home in Graham, Wash., in February 2012.

Authorities then turned their attention to her husband's brother, Michael Powell, 30, when they learned he had been made the heir to his brother's estate, which included the life insurance policies on Josh, his sons, and Susan that Josh had taken out.

Deputy Police Chief Powell said that while authorities looked at Michael Powell when Susan Powell was first reported missing, "there wasn't anything that jumped out initially" about him.

It wasn't until the summer of 2011, nearly two and half years after Susan Powell had disappeared, that police learned that Michael Powell had a car in a salvage yard in Pendleton, Ore. He had allegedly driven from Utah to Oregon in July, but the car broke down outside the city, where it was towed and left in the yard.

"We began to look at Michael Powell much more closely at that point," the deputy police chief said.

Police brought cadaver dogs to the place where Michael Powell's car had been impounded. While the car had not been crushed, according to the deputy police chief, only the frame and the structure of the vehicle remained.

"The dogs did indicate the odor of human decomposition [in the car]," said Deputy Police Chief Powell. "We were able to extract a minimal amount of DNA from the trunk of that vehicle."

While investigators could not obtain "a full profile" from the sample, they concluded the DNA did not match the profile of Susan Powell. Still, it made authorities interested in her husband's brother.

As the investigation progressed, Powell said police found a significant amount of communication between Josh and Michael Powell that had been encrypted via the Internet.

"We looked into him as completely and thoroughly as we possibly could," Deputy Police Chief Powell said. "I can tell you that he was involved in some capacity in the disappearance of Susan. There is a high probability that he had a direct involvement."